Dipping sauce

For gai yang, process garlic, coriander, lemongrass and pepper until finely chopped. Transfer to a non-reactive container with tamarind, palm sugar and fish sauce. Slash chicken at intervals with a large knife then add to tamarind mixture, turn to coat, rubbing with marinade, refrigerate to marinate (overnight).
Meanwhile, for dipping sauce, pound garlic, coriander root and chilli in a mortar and pestle to a paste. Transfer to a saucepan with vinegar, sugar and 200ml water and cook over high heat until sauce is syrupy (15-20 minutes). Refrigerate until required.
Preheat a coal barbecue or char-grill pan to high heat. Grill chicken, turning regularly, until charred and cooked through (25 minutes). Cut into portions and keep warm.
Meanwhile, drain glutinous rice and cook in a large steamer lined with muslin over a saucepan of boiling water until fluffy and sticky (20 minutes). Serve with chicken and dipping sauce.